This invention relates to a method for applying a releasing agent to a glass plate before subjecting the glass plate to hot bending working and an apparatus for same.
In producing a laminated and curved glass panel such as an automobile windshield it is often to simultaneously bend two glass plates before the lamination operation. That is, the two glass plates are placed one on top of the other without inserting any plastic interlayer therebetween, and in a suitably heated state the two glass plates are simultaneously and similarly bent, for example, by a press bending method. The thus bent two glass plates need to be separated from each other in advance of the lamination operation, but there is a possibility that the two glass plates adhere to each other by superficial fusion during the hot bending working. To prevent the two glass plates from adhering to each other it is known to apply a releasing agent to one of the two glass plate in advance. For example, gypsum, talc, calcium carbonate or mica is used as the releasing agent, and the releasing agent is applied to the glass plate as a dry powder or as a suspension or solution in a liquid, as described in JP No. 54-21847.
In the case of applying a releasing agent in the form of a dry powder to a glass plate it is not seldom that the powder is unevenly scattered over the glass plate surface so that the glass plate locally adheres to another glass plate when the two glass plates are provisionally laminated and simultaneously heated and bent. Besides, it is not easy to completely remove the releasing agent powder from each of the bent glass plates, and it is not rarely that the residual releasing agent turns into foreign matter which renders the glass plate defective. In the case of using a suspension of a releasing agent powder in a liquid the suspension on the glass plate surface must be dried to turn into a coating of a dry powder. Also in this case it is not easy to completely remove the powder from the glass plate after bending the glass plate.
It is possible to use a water soluble compound as the releasing agent, and it is relatively easy to form a uniform and relatively thin coating by applying an aqueous solution of the soluble releasing agent onto a glass plate and drying the liquid film on the glass plate surface. In this case the releasing agent can easily be removed from the glass plate by washing with water after bending the glass plate.
In conventional processes of producing laminated and curved glass panels with the use of a releasing agent, each glass plate is held horizontally while the releasing agent is applied thereto because this is convenient for the subsequent operations such as placing another glass plate on the glass plate coated with the releasing agent and then placing the two glass plates on a bending mold. However, during the application of the releasing agent it is not rarely that coarse particles of the releasing agent or particles of foreign materials fall onto the horizontally held glass plate from the top of the spraying booth or the environmental atmosphere and adhere to the glass plate. At the subsequent stage of bending provisionally laminated two glass plates the existence of a particle of foreign matter on the contacting surface of one glass plate is liable to result in the formation of a tiny concavity in either of the two bent glass plates. In the obtained glass panel such a concavity will cause local distortion of transmitted or reflected images as is called "spot distortion", and the glass panel will be judged to be defective.